1. Field of the Invention
The present invention relates to an image pickup apparatus for picking up the image of an object.
2. Related Background Art
In the field of the solid-state image pickup element, the cell size of the photoelectric conversion unit formed by the micro semiconductor process has been actively reduced in recent years for achieving the high resolution. On the other hand, in order to avoid the loss of the output electric signal converted in the photoelectric conversion unit, the solid-state image pickup element of amplifying type capable of amplifying and outputting the electric signal is attracting attention. Such solid-state image pickup elements of amplifying type include image sensors of MOS type, AMI type, CMD type, bipolar type etc.
Among these, the MOS solid-state image pickup element accumulates the photocarriers, generated by a photodiode, in the gate electrode of a MOS transistor, and outputs potential change of the gate thereof to an output unit after charge amplification according to a drive timing from a scanning circuit. Of such MOS type solid-state image pickup element, a device in which the photoelectric conversion unit and the peripheral circuits are all formed by a CMOS process is attracting particular attention.
FIG. 1 shows an internal configuration of a conventional solid-state image pickup element. In the solid-state image pickup element shown in FIG, 1, there are provided a photodiode 1 for converting the incident light into an electrical signal, a transfer switch 2 for transferring the signal charge from the photodiode 1 to a floating diffusion area 11, a source follower amplifier 10 (an amplifying transistor 10 and a constant current source 7) and a constant voltage source for amplifying and outputting the transferred signal, a selector switch 6 for reading the signal accumulated in a floating diffusion area 11 into a vertical signal line 13, and a reset switch 3 and a reset voltage source 4 for resetting the floating diffusion area 11 to a potential VPR.
The solid-state image pickup element shown in FIG. 1 is further provided with a vertical scanning circuit 14 for outputting control signals to the gates of the transfer switch 2, reset switch 3 and selector switch 6 for switching an on/off state thereof, transfer gates 15a, 15b for eliminating a fixed pattern noise generated by the switching of an on/off state of the transfer switch 2 etc., and a signal accumulating unit 16 for accumulating a signal which is input when the transfer gates 15a, 15b are turned on.
FIG. 1 shows an array of four pixels, but the number of pixels is not particularly limited, and the pixels may be arranged one-dimensionally or two-dimensionally.
FIG. 2 is a view showing a wave form of a control signal outputted from the vertical scanning circuit 14 shown in FIG. 1. In FIG. 2, there are shown a selection signal φSEL supplied to the gate of the selector switch 6, a reset signal φRES supplied to the gate of the reset switch 3, a transfer gate signal φTX supplied to the gate of the transfer switch 2, a noise signal readout signal φTN and a transfer signal φTS supplied to the gates of the transfer gates 15a, 15b. The gate of each switch is turned on or off respectively when each signal pulse is at the high level state or at the low level state.
In the following there will be explained, with reference to FIG. 2, the function of the solid-state image pickup element shown in FIG. 1. At first, low level signals are supplied to the gates of the selector switch 6 and the transfer switch 2, and a high level signal is supplied to the gate of the reset switch 3. In this state, the photodiode 1 executes photoelectric conversion of the incident light, and accumulates the obtained charge. The floating diffusion area 11 is maintained at a reset potential VPR.
Then the transfer gate signal φTX is shifted to the high level state to transfer the charge, accumulated in the photodiode 1, to the floating diffusion area 11 through the transfer switch 2. Then the transfer gate signal φTX is shifted to the low level state, whereupon the photodiode 1 executes the charge accumulation.
Then, immediately after the selection signal φSEL is shifted to the high level state and the reset signal φRES is shifted to the low level state, the noise signal readout signal φTN is shifted to the high level state to input the noise in the solid-state image pickup element into the signal accumulating unit 16 through the vertical signal line 13 and the transfer gate 15b. Then, after the noise signal readout signal φTN is shifted to the low level state, the selection signal φSEL is shifted to the low level state while the transfer gate signal φTX is shifted to the high level state, whereby the charge accumulation in the photodiode 1 is terminated and the charge accumulated therein is transferred to the floating diffusion area 11. Then the transfer gate signal φTX is shifted to the low level state to terminate the charge transfer from the photodiode 1 to the floating diffusion area 11 and to start again the charge accumulation in the photodiode 1. Since the charge accumulated in the photodiode 1 is transferred to the floating diffusion area 11 until the transfer gate signal φ TX is shifted to the low level state, the photodiode 1 is depleted and this state assumes an almost reset state.
Then the selection signal φSEL is shifted to the high level state and the transfer signal φTS is shifted immediately thereafter to the high level state, whereby an amplified signal, amplified by the source follower 10 based on the charge transferred to the floating diffusion area 11, is input into the signal accumulating unit 16 through the vertical signal line 13 and the transfer gate 15a. 
In the signal accumulating unit 16, the input signal input through the transfer gate 15b and the input signal input through the transfer gate 15a are so timed that the difference thereof can be calculated by an unrepresented differentiating circuit and are outputted to the output unit. The output unit calculates the difference of the two input signals thereby eliminating the fixed pattern noise generated at the switching of the on/off state of the transfer switch 2.
In the conventional technology, however since the content current source is connected to the vertical signal line, the potential thereof approaches 0 V during the charge accumulation in the photodiode. As a result, the potential of the vertical signal line becomes relatively low and there may result in a weak leak current from the vertical signal line to the photodiode.
Thus a charge injection takes place from the vertical signal line into the photodiode. Consequently, at the charge transfer from the photodiode to the floating diffusion area, a charge based on such leak current is superposed with the charge converted from the incident light, and an amplified signal based on such charges is read out to the vertical signal line.
As explained in the foregoing, the amplified signal thus read out may include the charge obtained by the actual photoelectric conversion and the charge resulting from the potential difference between the photodiode and the vertical signal line, and, in such case, the image based on such amplified signal may be deteriorated.